248 MOLLUSCA. CEPHALOPODA. Orbulita. 



96. A. rhotomagensis. — Discoid, radiated, umbilicated ; inner whorls partly 

 concealed ; radii about 20, furnished with three short tubercles upon the front, 

 and two, more or less elevated, upon each side ; whorls thick, with flattish 

 sides ; aperture oblong. Cuv. Oss. Foss. ii. 319. t. vi. f. 2. A. Sussexiensis, 

 Mantell. Suss. 114. t. xx. f. 2. t. xxi. f. 10. A. rhot. Sower. Min. Conch, t. dxv. 

 — Chalk-marl. 



97- A. Benettiana — Sower. Min. Conch, t. Dxxxix — No description as yet 

 given. 



98. A.biplicatus — Depressed, slightly umbilicate ; volutions inserted, trans- 

 versely radiated ; rays prominent, curved, bifurcated, arising from a row of 

 oblong projections on the inner edge of the volutions, and terminating in 

 tubercles on the outer margin ; carene flat, bordered by alternating, com- 

 pressed tubercles.— Mant. Suss. 91. t. xxii. f. 6 In blue chalk-marl. 



99. A. Woollgari — Discoidal, depressed, volutions one-third inserted, trans- 

 versely costated : costoe remote, slightly curved, inclined towards the aper- 

 ture, terminating on the outer margin, in compressed tubercles, or spinous 



projections; carene acute, deeply serrated Mant. Suss. 197- t. xxi. f. 16. 



t. xxii. f. 7 — Upper chalk. 



100. A. planorbis — Discoid, smooth ; whorls two or four, two-thirds exposed 

 Sower. Min. Conch, t. ccccxlviii — In Lias, at Walchel. Probably not of the 

 genus, or even of the group Cephalopoda. 



It is probable that not a few of the preceding species will, upon more ac- 

 curate comparison, be degraded to the rank of varieties. The following ob- 

 servation by Mr Sowerby merits attentive consideration. " There appears 

 to be no regular rule amongst Ammonites for their change of form, some be- 

 coming more globose, and others more compressed by age ; but they generally 

 lose some of the ornaments from their last whorls ; and, in their infant state, 

 are also smooth, or free from tubercles." t. 405. 



Gen. ORBULITA. — Sides equal, the last whorl embracing 

 and concealing the previously formed ones. 



1. O. disca. — Discoid, outer edge acuminated ; aperture sagittate, half 



the diameter of the shell in length, and one-sixth in breadth Ammonitis 



discus, Sower. Min, Conch, t. xii — In limestone of the Lower Oolite, Bed- 

 ford. 



2. O. striata — Discoid, gibbose, obscurely undulated, finely striated lon- 

 gitudinally; septa rather distinct, with four large angular folds. Aperture 

 semicircular, with nearly parallel edges ; siphunculus marginal ; a deep cen- 

 tral cavity. — Am. str. Sower. Min. Conch, t. liii. f. 1 In Carboniferous Lime- 

 stone, Derbyshire. In the Min. Conch, it is stated, at t. 130, that Dr Buck- 

 land has found this species in Transition Slate at Filliagh, near South Mol- 

 ton, Devonshire. 



3. O. sphcerica. — Orbicular, umbilicate ; septa with four broad angular 



folds ; aperture narrow ; syphon on the inner margin Conchy liolithus 



Nautilus sphsericus, Martin, Pet. Derb. t. vii. f. 3, 4, 5. Am. sph. Sower. Min. 

 Conch, t. liii. f. 2. — Carboniferous Limestone, Derbyshire. 



4. O. minuta. — Orbicular, with about 24 distant longitudinal striae ; aper- 

 ture lunate, rounded at the sides — Am. min. Sower. Min. Conch, liii. f. 3 



In Chalk Marl, Folkstone. 



5. O. modiolaris — Orbicular, central cavity large, with an angular edge, 

 exposing the inner whorls ; septa numerous, with five principal undulations, 



